Synthetic polymeric fibers that have physical and morphological characteristics generally similar to wood pulp fibers produced from natural woods have been known for approximately 10 years. Examples of such fibers are the synthetic wood pulp fibers formed of polyethylene that are sold by Crown Zellerbach under the trademark SWP.
Various methods of making synthetic wood pulp fibers are known, including (1) solution polymerization accompanied by stirring, (2) dissolving a preformed polymer and subjecting the solution to an anti-solvent, or (3) forming the polymer at the interface between liquid layers, with localized stirring provided to pull the polymeric material thus produced into fibrillated forms. Examples of methods of producing synthetic wood pulp fibers are disclosed in U.S. Pat. Nos. 3,560,318; 3,081,519; 3,003,912; 3,068,527; and 3,290,207; South African Pat. No. 697,431; United Kingdom Pat. No. 1,102,342; and Netherlands Patent Application No. Al32/48-7313178.
As used in this specification and the appended claims, the term "synthetic wood pulp fibers" means synthetic, water dispersible, thermoplastic, elongated, supple, randomly bent, polymeric fibers or fibrils generally similar in size and shape to conventional wood pulp fibers produced from naturally occurring woods. Each such "synthetic wood pulp fiber" is of irregular cross sectional shape measured at any given point along its length, and in addition is non-uniform in cross section along its length. The predominant shape of the fibers is usually rather ribbon-like.
The present invention utilizes synthetic wood pulp fibers in a high loft, low density, nonwoven fibrous material such as an air-laid web or fabric. Nonwoven materials are structures which consist of an assemblage or web of irregularly arranged fibers, joined randomly or more or less systematically by mechanical, chemical or other means. These materials are well known in the art, having gained considerable prominence within the last 20 years or so in the consumer market, the industrial commercial market and the hospital field. For example, nonwoven materials are becoming increasingly important in the textile and related fields, one reason being because of their low cost of manufacture for a given coverage as compared to the cost of more conventional textile fabrics made by weaving, knitting or felting. Typical of their use is the production of hospital caps, dental bibs, eye pads, dress shields, shoe liners, shoulder pads, skirts, hand towels, handkerchiefs, tapes, bags, table napkins, curtains, draperies, diaper facings, underpads, hospital drapes, and the like. Generally speaking, nonwoven materials are available today in a wide range of fabric weights of from as little as about 100 grains/sq. yd. to as much as about 4,000 grains/sq. yd. or even higher.
A number of processes and types of apparatus are known for producing nonwoven materials. These include (1) mechanical techniques (e.g. carding or garnetting), (2) wet laying techniques (e.g. inclined wire paper apparatus, cylinder paper apparatus, etc.), and (3) air-laying techniques. The high loft, low density, nonwoven materials such as webs or fabrics to which this invention relates may suitably be produced, in the manner to be explained in detail below, form layers of material manufactured by well-known air-laying processes.